1. So the necklace on his desk had to be the real diamond necklace.
2. So the necklace on his desk must have been the real diamond necklace.
(Does #1 mean #2? What is the difference between them?)
3. He must be strong.
4. He has to be strong.
(Are both grammatical and the same in meaning?)
Both 1 and 2 mean the same thing.
3 and 4 are the same in meaning, too, and are grammatically correct.
=)